1. Field of the invention
The present invention relates generally to the production of ophthalmic lenses, and, in particular pertains to a method and a device for removing molded soft contact lenses, high-precision intraocular lenses and the like, from the individual molds in which they are produced.
2. Discussion of the Prior Art
In view of the intense growth of the ophthalmic contact lens industry, it has become desirable and even necessary to be able to supply contact lenses which are periodically and frequently replaced in order to minimize the possibility of user induced contamination. This has created an opportunity for manufacturers to strive for automated methods and apparatuses that are able to automatically produce high quality ophthalmic lenses in a cost-effective and highly efficient manner.
It is currently the practice in the manufacturing technology for ophthalmic lenses, such as soft contact lenses of the hydrogel type, to form a monomer or monomer mixture that may be polymerized in a plastic mold. Details of typical direct mold processes for forming soft hydrogel contact lenses are described in U.S. Pat. Nos. 5,080,839, 5,039,459, 4,889,664, and 4,495,313. The process for forming soft contact lenses as generally described in the above-mentioned patents includes the steps of dissolving a monomer mixture in a non-aqueous, water-displaceable solvent and placing the monomer/solvent mixture in a mold having the shape of the final desired hydrogel lens. Thereafter, the monomer/solvent mixture is subjected to conditions whereby the monomer(s) polymerize, to thereby produce a polymer/solvent mixture in the shape of the final desired hydrogel lens. After the polymerization is complete, the solvent is displaced with water to produce a hydrated lens whose final size and shape are similar to the shape of the original molded polymer/solvent article.
Examples of typical plastic molds used for carrying the polymerizable feed material are disclosed in U.S. Pat. Nos. 5,094,609, 4,565,348 and 4,640,489. The mold disclosed in U.S. Pat. No. 4,640,489 is a two-piece mold with a female mold portion having a generally concave lens surface, and a male mold portion having a generally convex lens surface, both mold portions preferably made of a thermoplastic material such as polystyrene. As discussed in U.S. Pat. No. 4,640,489, polystyrene and copolymers thereof are preferred mold materials because they do not crystallize during cooling from the melt, and exhibit little or no shrinkage when subject to the processing conditions required during the direct molding process discussed above. Alternatively, it is also possible to employ molds made of polypropylene or polyethylene, such as described in U.S. Pat. No. 4,121,896.
During the molding process, the monomer and monomer mixture is supplied in excess to the female concave mold portion prior to the mating of the molds. After the mold portions are placed together, defining the lens and forming a lens edge, the excess monomer or monomer mixture is expelled from the mold cavity and rests on or between flanges that surround one or both mold portions. Upon polymerization this excess material forms an annular (HEMA) ring around the formed lens between the flange portions of the molds.
As discussed in the above-mentioned U.S. Pat. Nos. 5,039,459, 4,889,664, and 4,565,348, there is the requirement that the materials, chemistry, and processes be controlled so that the mold portions may be separated without having to apply an undue force, which may be necessary when the lens sticks to one or more of the lens mold or when the lens mold portions are adhered to each other by the excess HEMA ring after polymerization.
The prior art process for separating the mold portions and removing the lens therefrom consists of a heating stage, a mold half separation stage, and a lens removal stage. The heating stage of the prior art lens removal process is to apply heated air to the back mold portion thereby causing a differential expansion between the heated mold polymer and the cooler lens polymer. This differential expansion provides a shearing impetus which weakens the adhesion forces between the mold surface and the lens formed thereon. The mold half separation stage, which follows the heating stage is characterized by removal of the previously heated mold half. With respect to prior art systems for removing the back curve mold halves, inefficient means and damaging forces associated therewith have rendered such devices less desirable for producing high quality lenses, inasmuch as the steps of heating and separation that break the polymerized lens/polymer mold adhesion and provide access to the nearly formed lens occasionally damage the lens, and thereby decreasing the yield rate of the process.
With respect to the temperature gradient between the mold halves and the lens, the larger the thermal gradient, the more reduced will be the residual adhesion forces present between the lens and the mold halves, and correspondingly, the more reduced will be the force required to separate the mold portions. Conversely, the lower the thermal gradients created between the mold halves and the lens, the greater will be the required force to separate the mold portions. The greater the forces which may be required in separating the mold from the lens, the greater becomes the possibility of fracturing a mold portion and/or damaging the lens. Furthermore, it is to be understood that a process in which a thermal gradient must be applied on a repeated basis must be such whereby the environment does not heat appreciably, therein reducing the effectiveness of the process.
With respect to the separation of the mold halves, and thereby, the separation of the top mold half from the lens, it is understood that devices must be employed which do not damage, or apply undue stress on the contact lenses. When front and back curve mold parts, which are designed to form an integral frame such as are illustrated in U.S. Pat. No. 4,640,489, are placed together to form a lens shaped volume therebetween, the resultant combined structure provides limited accessible space for a separating means to engage and displace one mold from the other. Even minimal warpage of either mold half can adversely affect both accessibility to the space as well as the accuracy of the displacing forces.
The same requirements apply to the removal of the lens from the mold section in which it remains after separation.
Presently, as widely employed in the technology and as described in European Patent 0 775 571 A2 xe2x80x9cInfra-red Heat Source for Demolding Contact Lensesxe2x80x9d which is commonly assigned to the assignee of the present application, in order to assist in the demolding of the lens from the mold section there is employed infra-red heat source providing a thermal gradient wherein the infra-red energy is directed against the back curve of the mold through the intermediary of reflective tubes or buffers. In that instance, the structure as described that publication employs quartz or sapphire windows on the infra-red heater which filters out some of the infra-red radiation. This necessitates a longer period of heating and consequently lengthier demold times are required for demolding the lenses. Furthermore, pursuant to the foregoing construction, the infra-red heater employs one heater for multiple molds, in effect one heater for four molds, which in essence does not take into consideration potential variations in heat distribution among the various molds due to the presence of only a single infra-red heat output across the current sources for a plurality of molds.
Pursuant to another embodiment of the prior art, the thermal gradient which assists in the demolding of the lenses comprises the employment of a plurality of steam injection tubes each of which directs a jet of steam onto the concave surfaces of a back curve section. Pursuant to further variation described in the European patent publication, the thermal gradient is provided by a laser wherein a selected amount of concentrated, coherent light energy is directed at the back curve mold section, with the absorption thereof by the back curve providing the necessary thermal gradient.
In general, the process of providing the necessary thermal gradient, as indicated with the use of quartz or sapphire windows on the infra-red heater, filters out a portion of the infra-red radiation, and requires lengthier demolding times. In effect, the quartz glass surrounding the infra-red heating element is subject to breakage and causes an efficacy problem. Consequently, in order to protect the product from this problem, an additional hard protective window was required, in the nature of a sapphire protective window. However, the combination of the quartz element tube employed in the prior art and the sapphire protective window effectively attenuates the infra-red energy associated with these materials, thereby reducing the available output of the heat source and limiting the use of longer wavelength infra-red energy to excite the lens molds in order to derive the desired temperature gradients.
Accordingly, in order to further improve on the foregoing infra-red heat, or steam and laser devices employed to provide the necessary thermal gradient which assists in the demolding of the lenses, pursuant to the invention there is contemplated the provision of a novel infra-red radiation or heater device constituted of silicon carbide IR-emitters, and which employs an individual infra-red emitter for each individual mold, as opposed to the foregoing prior art construction which employ one infra-red heater for multiple molds.
The foregoing improvement also enables the use of an unfiltered infra-red emitter heating device whereby it is possible to achieve a precise control over the heating temperature for each individual mold rather than for conjointly a plurality of molds.
The elimination of the quartz and/or sapphire window on the infra-red heater which is utilized in the prior art also eliminates the filtering out of portions of the infra-red radiation, thereby providing a greater degree of efficiency by reducing the time of demolding required due to a greater portion of the generated infra-red radiation being received by the molds. Pursuant to the present invention, the independent control of the infra-red energy being emitted to each individual lens assembly facilitates the varying of the input wattage to each element or set of elements, whereby the magnitude in infra-red spectra profile can be readily adjusted in conformance with the requirements of each mold. In effect, higher wattages respond with high output in full wave distribution, whereas lower wattages respond with lower total output in a spectral shift away from short-wave infra-red emittance. Medium and long wave spectrum are more desirable for the demolding process.
In addition to the foregoing, pursuant to the invention there is also contemplated the provision of a preheating step in the production sequence prior to demolding of the lenses, in which a slight amount of heat in the process cycle preceding the demolding step utilizes any suitable heating source, such as an infra-red lamp, having a feedback control loop sensing the temperature at the demolding device, which enables thermal control over the molds to be within a specified temperature range upon entering the demolding station of the manufacturing system, thereby farther enhancing the efficiency of and reduction in demolding time.
Accordingly, it is a primary object of the present invention to provide an efficient and reliable means for applying a controlled thermal gradient to the unseparated mold sections, thereby providing a sufficient relative shear force to break the adhesion between the contact lens and the mold section.
It is another object of the present invention to provide a silicon carbide (SiC) IR-emitter heating device for demolding ophthalmic lenses that can easily and consistently separate the contact lens mold portions having a contact lens formed therebetween without damaging the lens.
Another object of the instant invention is to reduce contact lens manufacture process time by separating the greatest number of back curves from front curves in a rapid manufacturing line thereby permitting the fast and efficient production of hydrophilic contact lenses.
Pursuant to a more specific object of the present invention, a thermal gradient is provided at the demolding station through the provision of an infra-red emitter for demolding the lenses, wherein a ceramic infra-red emitter has a head portion constituted of silicon carbide enabling the emitting of unfiltered infra-red radiation without the need for a further quartz or sapphire window on the infra-red heater as required in the current technology.
Moreover, pursuant to a further aspect of the invention, it is an object to preheat the molds by means sensing the temperature at the demolding station and to relay the sensed information through a feedback loop to a preheater so as to ensure that the mold enters the demolding station while preheated to a specified temperature, thereby further decreasing the amount of time required for demolding.
Pursuant to another aspect of the invention, the inventive silicon carbide infra-red emitter which is utilized in the heating device for demolding the lenses employs a separate infra-red emitter for each separate lens mold, thereby enabling each mold to be thermally controlled so as to be within a specified thermal gradient required for the highly efficient and rapid demolding of the lenses.
The foregoing and other objects are attained by an apparatus for separating a back mold half from a front mold half of a contact lens mold assembly useful in the production of contact lens. Each of the front and back mold halves has a central curved section defining opposing concave and convex surfaces, and also has a circular circumferential flange which extends outward from the central portion. The concave surface of the front curve provides the shape defining surface of the front portion of the contact lens. Conversely, the convex surface of the back curve mold half provides the shape defining surface of the back portion of the contact lens. The fabrication of the contact lens, as set forth conceptually hereinabove, is carried out by placing a predetermined amount of monomer in the concave portion of the front curve, positioning the convex surface of the back curve mold section into the concave portion of the front curve mold section, and subsequently subjecting the monomer to curing or crosslinking, therein providing the lens shape to the hydrophilic material. The term xe2x80x9ccuredxe2x80x9d will be used herein to cover any reaction mechanism, including crosslinking, used to form the contact lens. The paired front and back curve mold sections may be transported through much of the fabrication line on pallets, each pallet containing a plurality of paired curve molds. Alternatively, the mold sections may be transported via another means, such as a conveyor or pusher rods, and may be transported individually or in plurals by such means. In the preferred embodiment the back curve rests on top of the front curve; however, the opposite is contemplated by this invention.
The mold separating and lens removal apparatus, which is positioned in the manufacturing line at a position downstream from the station wherein the lens material is cured, comprises a device for applying a thermal gradient to the concave surface of the back mold half, thereby providing a differential expansion which causes an adhesion breaking shearing force between the convex surface of the back mold half and the contact lens. As stated above, it is understood that the greater the thermal gradient, the greater the effectiveness of the adhesion breaking. Temperature gradient ranges from about 2.5 EC to 12 EC are desirable.
Pursuant to the invention as described in further detail hereinbelow, the required thermal gradient for assisting in the demolding of the lenses is provided by an infra-red heat source, this heat energy being directed at one of the mold curves through the intermediary of a silicon carbide (SiC) emitter heating device, whereby the device is constructed such that an individual silicon carbide infra-red emitter is associated with respectively each one of a separate mold of a plurality of molds which are transported to the demolding station on a pallet. In the preferred embodiment as described herein, the heat energy is directed at the back curve mold half, although the de-mold process would be as effective if the heat energy were directed at the front mold half instead of at the back mold half. This invention is therefore not limited to the application of heat to only the back mold half, and the front mold half could be substituted for the back mold half in the demold apparatus and method described below. However, the lens will remain adhered to the mold half that is not heated.
Pursuant to a further aspect, associated with the silicon carbide infra-red emitter which provides for the thermal gradient in the heating of the mold, there is provided a preheating step in the process, which incorporates a feedback control loop measuring the temperature sensed at the demolding station so as to raise each mold to within a specified temperature range upon entering the demolding station, such as within 57-65xc2x0 C., prior to the application of the demolding temperature gradient by the silicon carbide infra-red emitters.
Once the temperature gradient, supplied by the above device, has weakened the adhesion forces between the back curve mold sections and the corresponding lenses, an apparatus which is directed to complete the separation of the mold sections by mechanical means is introduced in the form of pry fingers between the front and back curve mold sections.
In one such embodiment, as described in EP 0775 571 A1, the separation apparatus comprises two pairs of opposing thin shims, oriented parallel to the direction of the advancing pallet, which are initially disposed on top of one another, and which together slide between the lateral extending flanges of corresponding front and back curves. Once so positioned, the upper ones of each pair shims is raised, therein lifting the back curve molds upward and away from the secured front curves and the lenses thereon. The removed back curves may be transported to a waste disposal area by a variety of devices, such as a plurality of suction cups. In a second variant, the separation device comprises an eccentric cam driven prying means mounted transverse to the direction of the advancing pallets as disclosed in U.S. Pat. No. 5,770,119 assigned to the assignee of the present invention. This prying means includes a first set of securing fingers which engage the front curve mold sections and hold them stationary as a second set of pry fingers, translated eccentrically, first pivotally and then substantially upwardly, engage the corresponding back curve mold halves. These prying fingers bias the back curve molds at a predetermined force with respect to the associated front mold halves, thereby effectively removing the back mold halves therefrom, and exposing the lenses. In a third variant, the separation device comprises a dual linkage, lifting device, mounted parallel to the direction of motion of the pallet stream that demolds the mold sections in pairs. This device includes thin retainer elements which slide between the flanges of the front and back curves as a pallet carrying the molds advances. The retainer elements secures the front curve mold sections to the pallet and prevents them from translating upward. As the front curve halves are secured by the retainer elements, a set of separation fingers, shaped for fitted engagement with the flanges of the back curve mold sections translates upward via a dual motion linkage system. The upward translation of the separation fingers lifts the back curves away from the stationary front curves and the pallet, thereby exposing the lenses, one pair of lenses at a time.